Effects of Past Global Change on Life (1995)
Chapter: Clastic Wetlands
preserved, often in exquisite detail, and range in size from tiny fragments to large axes. Coal balls occur in more than 65 upper Carboniferous coal seams, but may be confined largely to planar peats (Cecil et al., 1985).
Miospores are approximate indicators of the flora of coal forests. They can be recovered from most coals that have not been subject to intense metamorphosis because of the decay-resistant chemical composition of their outer walls. Correlation of spores with source plants (e.g., Courvoisier and Phillips, 1975; Willard, 1989) provides a baseline for inferring paleoecological patterns from palynological data. In this chapter we focus on the macrofossil (coal-ball) data base we have constructed, although palynological data will be referenced where appropriate.
Clastic Wetlands
The wetlands surrounding peat swamps were a typically diverse array of lowland settings including marshes, clastic (muddy) swamps, levees, splays, and channel margins, all part of larger flood basins. Most wetlands preserved in the fossil record were periodically flooded or had wet substrates (Scott, 1978; Gastaldo, 1987). The dominant plants included pteridosperms (seed ferns), ferns, and other groups that were locally important, such as sphenopsids (horsetails) or lycopsids. Such lowland areas encompassed a much wider array of habitats than did coal swamps. Consequently, there was much more potential for species migrations, habitat gradients, and generally broadly transitional boundaries (ecotones) between habitats, resulting in considerable floristic variation on basic themes.
Among the wetland habitats were swamps with large clastic influxes. These are often preserved as dark, organic shales. Many such swamps were lycopsid-dominated, like the peat swamps, and provide us with unusual information on spatial relationships of the plants, tree sizes, and gross morphology (Wnuk, 1985; DiMichele and Gastaldo, 1986; DeMaris, 1987; Wnuk and Pfefferkorn, 1987), important in the estimation of areal variability of peat-swamp assemblages (e.g., DiMichele and Nelson, 1989). In other cases, pteridosperm dominance of some clastic swamps and wet mineral substrates occurred in the Late Devonian and throughout the Carboniferous.
Plants of the clastic wetlands are generally preserved as compressions and impressions in relatively inorganic sediments, such as shales and sandstones, which provide external morphological features. Coal balls, on the other hand, preserve anatomical structure. Consequently, there are constraints in comparing taxonomic identifications from the two fossil forms. To the extent that identifying characteristics can be compared, the plants of peat deposits, and to a lesser degree those of flooded mineral-rich substrates, mostly represent different species from those in
Figure 8.1 Coal balls in situ; Herrin (No.6) Coal, Illinois. Large, light-colored masses are calcium carbonate permineralized coal balls, within the darker coal.
